1. Field of the Invention
This invention relates to transformed plants or algae with highly expressed chloroplast protein BPG2 (brassinazole-insensitive-pale green 2) or homologue thereof. The transformed plants or algae can produce higher levels of chlorophyll than wild type.
2. Background Art
Plant steroids, the brassinosteroids (BRs); brassinolide, castasterone, teasterone and so on, are essential for plant growth and development. The most active BR, brassinolide (BL), was first isolated from pollen of Brassica napus (Grove et al., 1979), and since then, more than 50 BRs have been isolated from other plant species (Bajguz and Tretyn, 2003). Molecular characterization of Arabidopsis BR biosynthetic mutants has revealed the important role of BRs in photomorphogenesis, leaf development, stem elongation, root elongation, pollen tube growth, xylem differentiation, sterility, and senescence.
deetiolated2 (det2) was first thought to be an abnormal photomorphogenesis mutant and was later identified as the first mutant deficient in BR biosynthesis (Chory et al., 1991). DET2 encodes a steroid 5α reductase involved in BR biosynthesis that can also catalyze mammalian steroid 5α reduction (Li et al., 1997; Fujioka et al., 1997). det2 has a dwarf phenotype with dark green round leaves and short inflorescences in the light, and a short hypocotyl and open cotyledons in the dark. In addition to these developmental characteristics, dark-grown det2 mutants also show increased expression of light-induced photosynthetic genes and their translated proteins encoded in the nuclear and chloroplast genomes. These results suggest that BR deficiency regulates chloroplast gene expression, as photosynthetic genes are normally not expressed in the dark. Based on the det2 phenotype, several BR-deficient mutants have been isolated, such as the BR biosynthesis mutants dwf4 (Azpiroz et al., 1998; Choe et al., 1998) and cpd (Szekeres et al., 1996) as well as BR-insensitive mutants such as the BR-signaling mutants bri1(Clouse et al., 1996; Li and Chory, 1997) and bin2 (Li et al., 2001; Li and Nam, 2002). These BR mutants generally show abnormal development in the light and de-etiolation in the dark. Previous characterization of the chloroplast in BR mutants has been limited, but it is important to further analyze the relationship between chloroplast development and BR.
Brz is a triazole compound that specifically inhibits BR biosynthesis by blocking the cytochrome P450 steroid C-22 hydoxylase encoded by DWF4/CYP90B1 (Asami et al., 2000, 2001). In the dark, Brz-treated Arabidopsis has open cotyledons and a short hypocotyl similar to BR-deficient mutants (Nagata et al., 2000). After growth in the dark for 40 days, plants treated with Brz develop true leaves with epidermal cells, guard cells, trichomes, palisade parenchyma cells, and spongy parenchyma cells. This phenotype in Arabidopsis can be rescued by addition of BR (Asami and Yoshida, 1999).
Recently, the mechanism of BR signal transduction in plant development has been analyzed in detail using chemical genetics to screen for mutants with altered responses to Brz in darkness at the germination stage. When grown in medium containing Brz, wild-type plants had short hypocotyls, but a mutant identified by the screen, Brz-insensitive-long hypocotyl1 (bil1-D) had a long hypocotyl in the dark (Asami et al., 2003). bil1-D has the same mutation as brassinazole-resistance 1-1D (bzr1-1D), and BZR1 encodes a functional transcription factor with dual roles in regulating BR biosynthesis genes and growth responses (Wang et al., 2002; He et al., 2005). BES1 was isolated from bri1-EMS-suppressor 1 (bes1-D), which is a semidominant suppressor of bri1. BES1 encodes a close homolog of BZR1/BIL1 but regulates BR response genes in plant development (Yin et al., 2002).
Here, we isolated and characterized a recessive Arabidopsis mutant, bpg2, which has pale green cotyledons and is insensitive to Brz-induced promotion of greening. BPG2 encodes a chloroplast protein that specifically regulates accumulation of 16S and 23S rRNA but not mRNA from the chloroplast genome. Brz-inducible protein accumulation in chloroplasts is suppressed by the bpg2 mutation. We have now found an important role of BPG2 in chloroplast development in BR signaling.
An object of this invention is to provide a method for producing a transformed plant or alga with increased chlorophyll.
Another object of this invention is to provide such transformed plant or alga.